1. Field of the Invention
The present invention relates to the filtering of a voltage in an arrangement in which the voltage is a stepped or rectangular voltage. The invention is particularly but not exclusively concerned with the filtering of a supply voltage in a switched mode power supply.
2. Description of Related Art
Modulated power supplies are used, for example, for providing a supply voltage to an amplification stage, typically a radio frequency (RF) amplification stage. An example of a particularly advantageous modulated power supply stage can be found in United Kingdom Patent No. 2398648.
In general, modulated power supplies provide a technique for tracking the supply voltage to an RF amplifier in dependence upon the RF input signal to be amplified by the amplifier. Such modulated power supply stages may typically be provided with a plurality of power supply voltages, one of which is selected in dependence upon a current level of the signal to be amplified. Thus there is provided a switching block which switches between one of a plurality of available power supplies to deliver a suitable power supply voltage to the RF amplifier.
In typical applications the output of the switching block is provided with a filter for filtering the selected voltage supply.
This filter gives rise to certain problems. Losses in the switching device may occur as a result of the filter input current being drawn through the “on” resistance of the switching devices. This input current comprises an unavoidable DC term due to the output load (e.g. the RF amplifier) being driven through the filter, and a “ripple” current determined by the filter input impedance.
In addition to these losses which occur in a switching device as a result of the connection of its output to the filter, there are also losses as a result of the filter itself.
These losses incurring in the filter and as a result of the filter cause variations to the output voltage delivered to the load from the filter. This is disadvantageous.
It is thus an aim of the present invention to provide an improved arrangement for filtering a stepped or rectangular voltage such as found in a modulated power supply.